Re: [PATCH 4/5] btrfs: qgroup: preallocate memory before adding a relation

[David Sterba <dsterba@suse.cz>]

On Thu, Jun 20, 2024 at 11:29:22AM +0930, Qu Wenruo wrote:
> 
> 
> 在 2024/6/20 02:39, David Sterba 写道:
> > There's a transaction joined in the qgroup relation add/remove ioctl and
> > any error will lead to abort/error. We could lift the allocation from
> > btrfs_add_qgroup_relation() and move it outside of the transaction
> > context. The relation deletion does not need that.
> >
> > The ownership of the structure is moved to the add relation handler.
> >
> > Signed-off-by: David Sterba <dsterba@suse.com>
> > ---
> >   fs/btrfs/ioctl.c  | 20 ++++++++++++++++----
> >   fs/btrfs/qgroup.c | 25 ++++++++-----------------
> >   fs/btrfs/qgroup.h | 11 ++++++++++-
> >   3 files changed, 34 insertions(+), 22 deletions(-)
> >
> > diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> > index d28ebabe3720..31c4aea8b93a 100644
> > --- a/fs/btrfs/ioctl.c
> > +++ b/fs/btrfs/ioctl.c
> > @@ -3829,6 +3829,7 @@ static long btrfs_ioctl_qgroup_assign(struct file *file, void __user *arg)
> >   	struct btrfs_fs_info *fs_info = inode_to_fs_info(inode);
> >   	struct btrfs_root *root = BTRFS_I(inode)->root;
> >   	struct btrfs_ioctl_qgroup_assign_args *sa;
> > +	struct btrfs_qgroup_list *prealloc = NULL;
> >   	struct btrfs_trans_handle *trans;
> >   	int ret;
> >   	int err;
> > @@ -3849,17 +3850,27 @@ static long btrfs_ioctl_qgroup_assign(struct file *file, void __user *arg)
> >   		goto drop_write;
> >   	}
> >
> > +	prealloc = kzalloc(sizeof(*prealloc), GFP_KERNEL);
> > +	if (!prealloc) {
> > +		ret = -ENOMEM;
> > +		goto drop_write;
> > +	}
> > +
> >   	trans = btrfs_join_transaction(root);
> >   	if (IS_ERR(trans)) {
> >   		ret = PTR_ERR(trans);
> >   		goto out;
> >   	}
> >
> > -	if (sa->assign) {
> > -		ret = btrfs_add_qgroup_relation(trans, sa->src, sa->dst);
> > -	} else {
> > +	/*
> > +	 * Prealloc ownership is moved to the relation handler, there it's used
> > +	 * or freed on error.
> > +	 */
> > +	if (sa->assign)
> > +		ret = btrfs_add_qgroup_relation(trans, sa->src, sa->dst, prealloc);
> > +	else
> >   		ret = btrfs_del_qgroup_relation(trans, sa->src, sa->dst);
> > -	}
> > +	prealloc = NULL;
> 
> This leads to memory leak, as if we're doing relation deletion, we just
> do the preallocation, then reset prealloc to NULL, no way to release the
> preallocated memory.

Right, I should maybe also add the preallocation only when it's adding
the relation so it does not fail for deletion where it's not needed.